1. Field of the Invention
The present invention relates to a high-frequency measuring device that detects high-frequency voltage and high-frequency current and measures that voltage and current by calculating high-frequency parameters such as an impedance, and to a calibration method for such a high-frequency measuring device.
2. Description of the Related Art
In the past, plasma processing systems have been developed that process processed articles such as semiconductor wafers or liquid crystal substrates using a method such as etching by supplying high-frequency electrical power output from a high-frequency power supply device to a plasma processing device. See, for example, Japanese Patent Application Laid-open Nos. 2007-163308 and 2004-309132.
FIG. 11 is a block diagram showing the configuration of a typical plasma processing system.
Since an impedance of a plasma processing device 400 fluctuates during plasma processing, there is the risk of a reflected power reflected at an input end of the plasma processing device 400 damaging a high-frequency power supply device 100. Thus, an impedance matching device 200 is typically provided in a plasma processing system A100 between the high-frequency power supply device 100 and the plasma processing device 400, and the impedance matching device 200 carries out a matching operation corresponding to fluctuations in the impedance of the plasma processing device 400. In addition, it is necessary to monitor the impedance of the plasma processing device 400 and high-frequency voltage and high-frequency current and the like at the input end of the plasma processing device 400 during plasma processing.
Monitoring of the plasma processing device 400 is carried out using various high-frequency parameters measured by a high-frequency measuring device 300 provided at the input end of the plasma processing device 400.
Together with the high-frequency measuring device 300 detecting high-frequency voltage (to be simply referred to as “voltage”) and high-frequency current (to be simply referred to as “current”) and determining a phase difference θ of the voltage and current from the detected values (to be simply referred to as “phase difference”), it also calculates high-frequency parameters such as an effective voltage value V, an effective current value I, an impedance Z=R+jX, a reflection coefficient Γ, a forward power Pf input to the plasma processing device 400, and a reflected power Pr reflected at the input end of the plasma processing device 400 due to impedance mismatch.
The high-frequency measuring device 300 has a capacitor capacitatively coupled to a rod-shaped semiconductor for transmitting electrical power to the plasma processing device 400 and a coil magnetically coupled to the body portion thereof, and detects a voltage v=√2·V·sin(ωt) with the capacitor or a current i=√2·I·sin(ωt+θ) with the coil. In addition, the high-frequency measuring device 300 determines the effective voltage value V, the effective current value I and the phase difference θ from a detected voltage v and current i, and then calculates the high-frequency parameters described above using these values according to the following equations (1) to (5). Namely, the high-frequency measuring device 300 is referred to as a so-called RF sensor provided with sensors for detecting the voltage v and current i, and an arithmetic processing circuit for calculating the high-frequency parameters from the detected values of those sensors.
                    R        =                              V            I                    ⁢          cos          ⁢                                          ⁢          θ                                    (        1        )                                X        =                              V            I                    ⁢          sin          ⁢                                          ⁢          θ                                    (        2        )                                          Z          =                      R            +                          j              ⁢                                                          ⁢              X                                      ⁢                                  ⁢                  Γ          =                                                                      (                                                                                    R                        2                                            +                                              X                        2                                            -                      1                                                                                                                (                                                      R                            +                            1                                                    )                                                2                                            +                                              X                        2                                                                              )                                2                            +                                                (                                                            2                      ⁢                      X                                                                                                                (                                                      R                            +                            1                                                    )                                                2                                            +                                              X                        2                                                                              )                                2                                                                        (        3        )                                Pf        =                              VI            ⁢                                                  ⁢            cos            ⁢                                                  ⁢            θ                                1            -                          Γ              2                                                          (        4        )                                Pr        =                  Pf          ⁢                                          ⁢                      Γ            2                                              (        5        )            
In general, since values detected with sensors differ from the correct values due to variations in sensor sensitivity, monitoring devices and measuring devices are typically composed to acquire calibration data that converts detected values to correct values by preliminarily measuring a measured object serving as a reference, and then correcting detected values to correct detection values with the calibration data during actual measurement.
Short-Open-Load-Thru (SOLT) calibration is used to calibrate the voltage v and current i detected by the high-frequency measuring device 300. SOLT calibration consists of first connecting the high-frequency measuring device 300 to a standard having a preliminarily specified true value of an impedance, and then measuring the impedance with the high-frequency measuring device 300. A dummy load having a characteristic impedance of the measurement system (characteristic impedance of a transmission line that transmits high-frequency waves for measurement, and typically an impedance of 50 or 75Ω) and dummy loads having an impedance close to each of an open-circuit impedance (an infinitely large impedance) and a short-circuit impedance (a zero impedance), are used as standards. Next, calibration parameters for calibrating the voltage v and the current i calculated from an impedance of each standard measured by the high-frequency measuring device 300 and a true value of the impedance of each standard, and then recorded in memory (not shown) of the high-frequency measuring device 300. During actual measurement, each high-frequency parameter is calculated after having corrected the detected voltage v and current i with the calibration parameters recorded in memory.
However, since the above-mentioned calibration parameter was determined so as to be able to carry out calibration over an extremely wide range of impedance, the accuracy of calibration according to this calibration parameter is not sufficiently high. In addition, in the calibration described above, an impedance is measured by directly connecting the high-frequency measuring device 300 to each standard, and the calibration parameter is calculated based on this measured impedance. In other words, this calibration parameter calibrates various high-frequency parameters at an output end of the high-frequency measuring device 300. Thus, an effective voltage value V and effective current value I that have been measured after calibrating in this manner are the effective voltage value and effective current value at the output end of the high-frequency measuring device 300, while the impedance Z is an impedance as viewed from the output end of the high-frequency measuring device 300 towards the load side. However, various high-frequency parameters are required to be measured within the chamber of the plasma processing device 400 in order to monitor the plasma processing device 400.
In the case of directly connecting the high-frequency measuring device 300 and the plasma processing device 400, although the effective voltage value, the effective current value and the impedance at the output end of the high-frequency measuring device 300 can be considered to be equivalent to the effective voltage value, the effective current value and the impedance within the chamber of the plasma processing device 400, in the case of installing the high-frequency measuring device 300 and the plasma processing device 400 separated by a distance there between, the effect of parasitic capacitance of a transmission line (such as a coaxial cable) between the high-frequency measuring device 300 and the plasma processing device 400 becomes large, thereby preventing monitoring of the plasma processing device 400 with the high-frequency measuring device 300.
For example, measured values of the high frequency measuring device 300 may be used for an E chuck controller. An E chuck controller controls the strength of an electrostatic chuck for immobilizing a wafer in the chamber of the plasma processing device 400 based on measured effective current and effective voltage values. Thus, necessary to measure effective voltage and control values within the chamber with high accuracy. However, in the case of installing the high-frequency measuring device 300 and the plasma processing device 400 separated by a distance there between, it is difficult to control the strength of the electrostatic chuck based on effective voltage and current values measured by the high-frequency measuring device 300. Moreover, since plasma electron density is closely related to electrode voltage and current, it is still necessary to accurately measure effective voltage and current values even in the case of estimating plasma parameters.